As liquid crystal molecules do not produce light themselves, an external light source is needed in conventional LCDs to display pictures. Backlight sources are categorized into edge-lit type backlight and direct-lit type backlight, based on the relative disposition of the backlight sources. In an edge-lit type backlight configuration, the light source is disposed at a side of the light guide plate (LGP) and light from the lamp bar is directly emitted into the LGP. The LGP is mainly for transforming parallel light incident from the edge into a surface light. In a direct-lit type backlight configuration, the light source is arranged below the liquid crystal panel and the light from the light source travels through a certain distance spatially and is diffused and mixed by the diffuser plate to exit as a surface light source.
Currently, the direct-lit type backlight plays an important role in the LCDs. In the direct-lit type backlight unit, a light-emission surface of a LED is faced with the diffuser plate directly, which can easily produce a light spot with different brightness on the surface of the backlight. To improve the uniformity of the surface light source, a distance between the light source and the film plate is increased to diffuse the light sufficiently (that is, to increase the light mixing height), which will leads to relatively thick backlight unit. To overcome disadvantages of the direct-lit type backlight unit, some conventional technology adds a secondary optical lens in the backlight unit such that the light is diffused through the lens. However, such a design requires a secondary optical system and has too many optical interfaces, thereby limiting the efficiency and the volume of the LED module.
Moreover, picture quality of a LCD panel is directly related to the flatness of optical films in the backlight unit. In the direct-lit type backlight unit, a separate support frame is arranged between the backplate and the diffuser plate to achieve a flat film. However, such a support frame may negatively affect the optical performance of the backlight unit as well.